c++ - 在 C++14 中，constexpr 成员可以更改数据成员吗？

Question

在 C++14 中，由于 constexpr 不再是隐式 const，constexpr 成员函数是否可以修改类的数据成员:

struct myclass
{
    int member;
    constexpr myclass(int input): member(input) {}
    constexpr void f() {member = 42;} // Is it allowed?
};

Answer 1

是的，我相信这个变化是从 proposal N3598: constexpr member functions and implicit const 开始的并最终成为 N3652: Relaxing constraints on constexpr functions 的一部分这改变了部分 7.1.5 段落 3 白名单中函数体中允许的内容:

its function-body shall be = delete, = default, or a compound-statement that contains only

• null statements,
• static_assert-declarations
• typedef declarations and alias-declarations that do not define classes or enumerations,
• using-declarations,
• using-directives,
• and exactly one return statement;

加入黑名单:

its function-body shall be = delete, = default, or a compound-statement that does not contain

• an asm-definition,
• a goto statement,
• a try-block, or
• a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.

并且还在 C.3.3 部分第 7 条:声明中添加了以下注释:

Change: constexpr non-static member functions are not implicitly const member functions.

Rationale: Necessary to allow constexpr member functions to mutate the object.

Effect on original feature: Valid C++ 2011 code may fail to compile in this International Standard. For example, the following code is valid in C++ 2011 but invalid in this International Standard because it declares the same member function twice with different return types:

struct S {
 constexpr const int &f();
 int &f();
};